The present invention relates generally to the field of inspection of a solid object by a charged particle beam, and more particularly to techniques to reduce the effect of the charged particle beam on the object being inspected.
Microelectronic fabrication requires accurate measurement of features in a microelectronic structure to ensure proper process control. A typical measurement technique uses a scanning electron microscope (SEM) to measure structural features, typically surface features that may be accessed by the electron beam of the SEM. In an SEM, accelerated electrons are tightly focused into a beam that is used as a measurement probe for the microelectronic structures, and the beam is scanned across the feature to be measured. When the electron beam is incident on a structure, the electrons interact with atoms in the structure to produce various signals that can be detected and that contain information about the structure's surface topography and composition. A blanking signal to the SEM is used to interrupt the electron beam incident on the structure. This may be accomplished, for example, by interrupting power to the electron emitter, deflecting the electron beam, blocking the electron beam, or any other suitable method.
A common method of SEM measurement is based on the detection of secondary electrons emitted by atoms excited by the electron beam. Secondary electrons that exit the structure in any direction may be captured, accelerated, and detected by a scintillator and photomultiplier arrangement. While secondary electrons are emitted in all directions, they are more concentrated in the direction of the beam. Thus, for example, an electron beam that is incident perpendicular to a flat surface will result in fewer detectable secondary electrons than an electron beam that is incident on a tilted surface. Similarly, an electron beam that is incident on a concave surface, for example, where a feature formed on top of a surface meets the surface, will result in fewer detectable secondary electrons. This property of secondary electron emission makes SEMs useful in detecting feature “edges”, as outside edges tend to produce larger secondary electron currents relative to neighboring flat portions, and inside edges tend to produce smaller relative secondary electron currents. An edge generally may be any topographical feature of the microelectronic structure that is topographically distinguishable from its surrounding topography. As the electron beam is scanned across the feature, changes in the topography of the feature along the scan path may be detected. Correlating the beam position to positions on the microelectronic feature may allow for high precision measurements of various feature characteristics.